What is a gravitational wave? It’s a ripple in the fabric of space and
time. Imagine that space is a giant sheet of rubber: things that have mass cause that rubber sheet
to bend, like a bowling ball on a trampoline. The more mass, the more that space gets bent
and distorted by gravity. For example, the reason the earth goes around
the sun is that the sun is very massive, causing a big distortion of the space around it. If you just try to move in a straight line
around such a big distortion, you will find yourself actually moving in a circle. That’s how orbits work: there’s not an
actual force pulling the planets around, just a bending of the space. Gravitational waves are produced whenever
masses accelerate, changing the distortion of space. Everything with mass and/or energy can make
gravitational waves. If you and I started to dance around each
other, we would also cause ripples in the fabric of space and time. But these would be extremely small. Practically undetectable. Now gravity is very weak in the scale of the
other forces in the Universe, so you need something really, really massive moving very,
very fast, to make the big ripples that we can detect. How would you observe a ripple in space? If the space between you and me stretched
or compressed, we wouldn’t notice it if we had made marks on our metaphorical rubber
sheet, for example, using equally spaced rocks. Because these marks would also get stretched
further apart. But there is one ruler that doesn’t get
stretched, one made using the speed of light. If the space between two points gets stretched,
then light will take longer to go from one point to the other. And if the space gets squeezed, light takes
less time to cross the two points. This is where the LIGO experiment comes in. It has 4 kilometer long tunnels and uses lasers
to measure the changes in the distance between the ends of the tunnels. When a gravitational wave comes through, it
stretches space in one direction, and squeezes space in the other direction. By measuring the interference of the lasers
as they bounce between the different points, physicists can measure very precisely whether
the space in between has stretched or compressed. And the precision needed is incredible. To detect a gravitational wave, you need to
be able to tell when something changes in length by a few parts in 10 to the 23. It’s like being able to tell that a stick
one sextillion meters long has shrunk by 5mm. The effect of a Gravitational Wave is so minuscule
and easily confused with random noise you need a smart data analysis technique. Scientists hope to identify the patterns of
gravitational waves by comparing the wiggles they measure in the experiment to the wiggles
they expect from Gravitational Waves. That’s like trying to identify a song being
hummed at a noisy party. A very, very noisy party. Imagine that your whole life you had been
deaf until one day your hearing was restored. You’d be able to explore the Universe in
this whole new way. That’s why detecting gravitational waves
is so significant. It’s a completely new way of studying the
Universe. Anytime there's a new way to observe the Universe
we discover things that we didn't expect. It's really about looking for new things that
we didn't know existed, examining the extreme edges of our knowledge of physics and testing
our theories about how the Universe works.
